Carvones hydrogenation

Dispersion, particle size and activity of Rh catalysts for carvone hydrogenation. 

The results of Table 1, show that the preparation method does not affect the metallic dispersion. However, the catalysts prepared in ammoniacal solution have the lowest activity per site, showing that in carvone hydrogenation an important precursor effect in activity is obtained. 

Stereoisomers selectivity (%) of carvomenthol of carvone hydrogenation on Rh Catalysts. 

Selectivity (54) for carvone hydrogenation on Rh catalysts prepared from carbonyl clusters. 

The following important conclusions emerge from this study (i) precursor effect is exhibited in carvone hydrogenation activity, (ii) the Rh/MgO catalysts results to be more selective towards carvotanacetone formation than... 

Figure 1 shows the effect of solvent on the rates of carvone hydrogenation over the homogeneous Wilkinson s catalyst and the different supported... 

The initial selectivity of platinum catalysts in carvone hydrogenation depends on the platinum dispersion. For example carvotanacetone (2-methyl-5-isopropylcyclo 2-ene-l-one) is the major product on catalysts with large platinum particles, whereas carvomenthone (2-methyl-5-isopropylcyclohexanone) is the major product on highly dispersed platinum catalysts. 

The aim of this work is to study the effect of Pt dispersion and the addition of a second metal (Au) in the selective carvone hydrogenation. 

The metal accesibiMty of the catalysts was determined by gas chemisorption (hydrogen, oxygen) at room temperature in a conventional volumetric equipment. The widely admitted adsorption stoichiometries are H/Pt = 0/Pt = 1. No chemisorption of hydrogen and oxygen on gold was observed at this conditions (14). The carvone hydrogenation reaction, in... 

Activities per site for the carvone hydrogenation on the various monometallic catalysts are shown in Table 2. It can be observed a particle size effect large particles (Cl) are more active than smaller ones (Al). This particle size effect is also observed in the selectivity pattern. Partial hydrogenation (exo double bond) is the main reaction on large particles, the main product being carvotanacetone. On the other hand, small particles are more selective towards carvomenthone, the two double bonds (exo, endo) are hydrogenated. 

Table 2 Activity (TOP) and Selectivity on Pt/SiO catalysts for carvone Hydrogenation...

Table 3 Activity and Selectivity of Pt-Au/SiOa catalysts for carvone hydrogenation...

On the other hand the change in selectivity for carvone hydrogenation could be explained by assuming that the atoms of Au are deposited preferentially on edges, corners, leaving free large planes, on which a rapid desorption of carvotanecetone can occur. 

In conclusion, the change in selectivity in carvone hydrogenation on Pt-Au/SiOa catalysts prepared by Redox reaction can be explained in terms of a preferential deposition of Au on low coordination sites as it was pointed out by local microanalysis. 

Absorption, metaboHsm, and biological activities of organic compounds are influenced by molecular interactions with asymmetric biomolecules. These interactions, which involve hydrophobic, electrostatic, inductive, dipole—dipole, hydrogen bonding, van der Waals forces, steric hindrance, and inclusion complex formation give rise to enantioselective differentiation (1,2). Within a series of similar stmctures, substantial differences in biological effects, molecular mechanism of action, distribution, or metaboHc events may be observed. Eor example, (R)-carvone [6485-40-1] (1) has the odor of spearrnint whereas (5)-carvone [2244-16-8] (2) has the odor of caraway (3,4). 

By reduction carvone fixes 2 atoms of hydrogen on to the ketonic group, and 2 atoms in the nucleus, with the formation of dihydrocarveol, CijHjgO, whose corresponding ketone, dihydrocarvone, Cj Hj O, exists in small quantities in caraway oil. 

G. Vavon has examined the hydrogenation of carvone, in presenc of platinum black as a catalyst, and shown that it takes place in three entirely distinct phases. Carvone fixes successively three molecules of hydrogen, giving dextro-carvotanacetone, then tetrahydrocarvone, and finally carvomenthol. 

Carvone, a substance responsible for the odor of spearmint, has the following structure. Tell how many hydrogens are bonded to each carbon, and give the molecular formula of carvone. 

Table 3 summarizes the scope and limitation of substrates for this hydrogenation. Complex 5 acts as a highly effective catalyst for functionalized olefins with unprotected amines (the order of activity tertiary > secondary primary), ethers, esters, fluorinated aryl groups, and others [27, 30]. However, in contrast to the reduction of a,p-unsaturated esters decomposition of 5 was observed when a,p-unsaturated ketones (e.g., trans-chalcone, trans-4-hexen-3-one, tra s-4-phenyl-3-buten-2-one, 2-cyclohexanone, carvone) were used (Fig. 3) [30],... 

Equation (81)), while the other two C=C double bonds in the structure are intact. Under the same reaction conditions, the racemic carvone is also resolved kinetically with a KR/KS ratio of 33 1. Asymmetric hydrogenation of a,/Tacetylenic ketones to chiral propargylic alcohols is still unavailable. 

The Wilkinson catalyst reduces external double bonds much faster than the internal ones as in the hydrogenation of carvone (equation l)7. 

The hydrogenation of the monoterpenes (—)- and (-l-)-carvone was studied extensively. Several microorganisms were used in these reductions. They catalyzed the production of all possible stereoisomers, but some of them only in small quantities. The distribution of the products depended on the catalyst applied116. 

Literature examples include one-, two-, and three-step hydrogenation reactions, hi each case, the reactant and product concentrations were monitored, as were intermediates or side reaction products as necessary. Simple peak height or area measurements were sufficient to generate reaction profiles for the reduction of cyclohexene [116] and of l-chloro-2-nitrobenzene [117]. However, for more spectrally complex systems, such as the reduction of carvone and of 2-(4-hydroxyphenyl) propionate, multivariate curve resolution (MCR) was required [117]. 

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